The present invention relates to a projection device configured to obliquely project an image formed on an image source to a screen using a trapezoidal intermediate image.
Conventionally, a projection device that obliquely projects an image displayed on an image source unit onto a screen has been known. It is noted that, in the following description, a term “projection device” represents the obliquely displaying type project device as described above.
Generally, the projection device is configured such that an image source displays an image to be projected on the screen. The image is typically a rectangular shape with a predetermined aspect ratio. Using a first optical system, light carrying the image displayed by the image source is converged on an intermediate image plane. It is noted that the light is obliquely incident on the intermediate image plane, and the image formed on the intermediate image plane has a trapezoidal shape. Next, the image formed on the intermediate image plane is projected on the screen using a second optical system. The light is also incident on the screen obliquely such that the trapezoidal shape of the intermediate image is re-shaped and a rectangular image is formed on the screen.
Further, the image source, the first optical system and an intermediate image plane are arranged to satisfy Scheinpflug's law. Similarly, the intermediate image plane, the second optical system and the screen are arranged to satisfy Scheinpflug's law. With such a configuration, the image displayed on the image source unit can be displayed on the screen by obliquely incident light carrying the image with focused condition.
An example of such an projection device is disclosed in Japanese Patent Provisional Publication No. HEI 06-265814 (hereinafter, referred to as '814 publication).
In '814 publication, a real image reducing the image displayed on the image source unit is formed as the intermediate image, and the real image is obliquely projected on the screen with a screen side optically system. According to '814 publication, however, since a reduced intermediate image is formed, if dust or small particles invade on an image plane of the intermediate image, the quality of the image is significantly deteriorated. Further, since heat concentrates in the vicinity of the intermediate image plane, deterioration of the optical system itself may be accelerated. Further, due to the heat, a plastic lens, which is considered to have high design freedom, cannot be used as optical elements (e.g., lens) arranged in the vicinity of the intermediate image plane.
Furthermore, in order to form a reduced intermediate image, it is necessary to decrease an F-number of the image source side optical system. In such a case, a difference of incident angles of rays incident on an element in the vicinity of the intermediate image at both side ends of the element increases. Due to this angular difference, refraction angles of rays incident on the optical element are uneven, which causes aberration. That is, the configuration of decreasing the F-number deteriorates optical performance of the image source side optical system.